Photographic compositions and elements



United States Patent 3,155,514 PHGTQGRAPEEC COMPOSlTiN AND ELEMENTSRalph Kingsley Blake, Westfield, N.J., assignor to E. l.

du Pont de Nemours and Company, Wilmington, Del,

a corporation of Delaware No Drawing. Filed Nov. 8, 1962, Ser. No.236,412

6 Claims. (Cl. 96-4167) This invention relates to photography, and moreparticularly to new photographic products useful therein.

The principal processes of photography are based on the use ofcolloid-silver halide emulsion layers. In the prior art processes alatent image is formed by image-wise exposure of a radiation-sensitivesilver halide emulsion layer. Silver halide bearing a latent image hasbeen developed to silver by selective reduction in these instances.

In the prior processes of photography the unreducecl silver remainingafter development has been removed by silver halide solvents or renderedinsensitive or transparent by treatment with complexing agents. Optionalaftertreatrnents include intensification and reduction, toning andtinting. However, the primary or first step in image formation alwayshas been based on the selective reduction step.

It is an object of this invention to provide new photographic silverhalide compositions, photographic layers and photographic elementsbearing a layer of silver halide. Another object is to provide processesfor making these products. A further object is to provide such productswhich are adapted to more versatile processes for forming silver andother images and which are simple, dependable and give results equal inquality to the prior conventional methods. Still further objects will beapparent from the following description of the invention.

The objects of this invention are realized by new silver halidecompositions in which an image may be formed, after exposure to actinicradiation, by imagewise solution of the silver halide. The residualsilver halide image may then be converted to silver, dyed or tonedimages.

The novel photographic emulsion layers and elements of this inventioncomprise, before exposure to actinic light, silver halide crystalshaving associated therewith, in substantially greater thanfog-inhibiting amounts, a silver mercaptide of a substituted thioureahaving at least one SH group in one of its tautomeric forms, the lattercompouind being characterized in that when admixed with an aqueoussilver halide dispersion it protects the silver halide crystals to suchan extent that when a silver halide dispersion protected by saidcompound is treated with 10%, by weight, aqueous sodium thiosulfate atleast three times the amount of silver halide remains undissolved as ina similar dispersion successively treated with aqueous sodiumhypochlorite and sodium thiosulfate, after vigorous agitation of thedispersions for 30 seconds at C.

Preferably, the silver halide crystals are dispersed in awater-permeable organic colloid to form a 1ight-sensitive photographicemulsion. The selected substituted thiourea compound can be added to thesilver halide emulsion while the latter is in the liquid state or theemulsion may be coated on a suitable support and the resulting elementbathed or impregnated with a solution, e.g., an alcoholic solution ofthe organic compound. In the working examples below, the amount oforganic compound in the silver halide emulsion is from about (3.4 to 63g. per mole of silver halide but wider ranges of concentration can beuseful, depending upon the particular organic compound, the size andnature of the silver halide crystals, the presence of other materialswhich may partially cover the Patented Nov. 3, 1964 surface of thesilver halide crystal, and upon various other factors.

The gelatimsilver halide ratio is quite flexible and may vary from 3:1to 1:30 depending on the particular organic compound and application.

In one commercially practical aspect of the invention, the silver halideis present in much higher concentration than in conventional emulsionsand emulsion layers.

In an important use of the products of the invention, direct positiveimages are formed by a process which comprises (a) Exposing imagewise toactinic radiation a photosensitive layer comprising silver halidecrystals treated with the organic compound as described above.

(b) Treating the exposed layer in a solution of a silver halide solventto remove soluble silver halide in the exposed image areas, thus forminga positive silver halide image, and

(c) Washing the resulting layers.

If desired, the silver halide image may be viewed directly, e.g., byprojection (if on a transparent support) or it may be intensified by s(d) Converting the residual silver halide to silver by treatment in afogging developer, e.g., a high pH, l-phenyl-4-methyl-3-pyrazolidone/hydroquinone developer containing iodide ion orby fogging the emulsion by exposure to light and then treating with asilver halide reducing agent, e.g., a conventional silver halidedeveloper, and

(e) Washing the developed layer to reveal a positive silver image in theoriginal non-exposed areas.

The imagewise solution of the exposed silver halide/organic compoundstratum may be effected by the silver halide solvents commonly used asphotographic fixing agents, e.g., sodium thiosulfate, sodiumthiocyanate, concentrated solutions of potassium bromide, etc. Reductionof the treated, residual silver halide may be accomplished by use of anychemical reducing agent capable of reducing silver ion to silver metal,e.g., hydroquinone, metal sodium hydrosulfite and stannous chloride. Thefunction of the reducing agent may be enhanced by modifying the surfaceproperties of the treated, residual silver halide crystals by means ofalcohol, thiourea, potassium iodide, etc. The silver halide image may betoned, e.g., with sodium sulfide, sodium selenide, etc. In addition,color images may be obtained by developing the treated, residual silverhalide with a primary aromatic amine color developing agent in thepresence of a color coupling compound either in the developing bath orpreviously in corporated in the emulsion.

The present invention embodies a broad new photographic principle and isnot limited to a narrow class of thiourea compounds with which thesilver halide crystals are intimately associated or may be treated inpreparing the novel compositions of this invention. Instead, a largenumber of useful compounds can be used and their utility can be readilydetermined by a relatively simple test. Essentially, the test consistsof two steps, Test A and Test B. In Test A, the candidate thioureacompound must render a dispersion of silver halide crystals insoluble ina silver halide solvent, i.e., an aqueous solution of sodiumthiosulfate, at some pH between 1 and 13. If the candidate compoundmeets the insolubility requirements of Test A, it must also meet therequirements of Test B by forming with said dispersion of silver halidecrystals a reaction product which, upon treatment with an aqueous ofsodium hypochlorite, becomes soluble when subse quently treated withaqueous sodium thiosulfate. The following practical tests are providedin further exemplification of the invention and include specificconcentrations of solutions, times, etc., so that suitable organiccompounds may be readily and positively identified.

3 resr A A solution nearly saturated at 25 C. with a candidate organiccompound is prepared using ethanol, acetone, dimethyl formamide, Wateror other suitable solvents. De-

ending on the solubility, a solution concentration from 0.01 to percentby weight is obtained. T wenty-five ml. of a silver chlorobromidedispersion containing mg. of silver halide (calculated as silverbromide), prepared as described below, is treated with small increments(i.e., about 0.1 to 0.2 ml. at a time) of the said candidate solutionunder safelight conditions (Wratten 1A filter or equivalent) until thesilver halide dispersion either is rendered insoluble in 10% aqueoussodium thiosulfate or the candidate is found not to causeinsolubilization. Generally insolubilization will occur upon theaddition of 0.05 g. or less of said candidate compound, calculated asthe pure compound. Compounds which must be used in substantially greaterquantities than this, e.g., 1-2 g. to effect insolubilization areconsidered less preferred compounds. The silver halide dispersioninsolubility is determined by taking a 0.5-ml. portion of the silverhalide dispersion (after each incremental addition of the candidateorganic compound), adding about 0.1 to 0.2 ml. of 10% aqueous sodiumthiosulfate solution and observing the turbidity after seconds.

As a control, one should use 25 ml. of water to which small incrementsof the candidate solution are added. Half milliliter portions of thecontrol are treated in the same manner with the sodium thiosulfatesolution. The presence of visual turbidity relative to the control issufficient to satisfy the definition of insolubility in this test.

This test may be repeated for various pH increments from 1 to 13.Although there is some optimum pH value at which the test is mostsensitive, this is not a sharp maximum which must be precisely attained.Rather, it has been found that there is a fairly broad range of pHvalues (e.g., 2.0 to 3.0 pH units) over which the test has asatisfactory sensivity. In practice, the silver halide dispersion mightbe tested without adjustment (e.g., at pH 5.0 to 7.0) and ifinsolubilization occurs here, Test A is completed. If there is noinsolubilization, the test is repeated at a higher pH (e.g., from pH10-13). if there is still no insolubilization, the test is conductedwith emulsion adjusted to a lower pH (e.g., about pH 1-3). Thus threedifferent pH values represents a practical maximum number which must beinvestigated to determine whether or not insolubilization will occur.

TEST B An organic compound capable of insolubilizing a silver halidedispersion according to Test A is now ready for the next test, whichagain will be conducted under safelight conditions. To the above silverhalide dispersion, there is added the minimum amount of a solution ofthe candidate organic compound found necessary for insolubilization.Half-milliliter samples of the dispersion containing 0.5 mg. AgBr or2.29 mg. Ag are placed in two test tubes. To one sample is added 0.5 ml.of water; to the other is added 0.5 ml. of a 5% by weight aqueoussolution of sodium hypochlorite (containing 25 mg. sodium hypochlorite).Next, there is added to both samples, 1.0 ml. of an aqueous 10% byweight solution of sodium thiosulfate (containing 100 mg. sodiumthiosulfate). If, after standing for up to thirty seconds, the sampletreated with sodium hypochlorite clarifies (or becomes less turbid)relative to the control sample, the candidate organic compound meets therequirements of Test B and is satisfactory for use in accordance withthis invention.

Silver Halide Dispersion Prepm'ati0nDispersion I A silver halidedispersion useful for Tests A and B is prepared according to thefollowing specifications. In red light, 30 g. of photographic gradegelatin is soaked in 1100 ml. of distilled water for 10 minutes. Thetemperature is then raised to 120 F. and g. of solid ammonium chlorideadded. The mixture is stirred at F. and after the ammonium chloride iscompletely dissolved, a solution made by diluting 500 ml. of 3 N silvernitrate with 2000 ml. of distilled water is added while stirring thesolution for 5 seconds. This mixture is held at 120 F. for 4 minuteswith stirring, and then ml. of 3 N ammonium bromide added (30 molepercent) in 10 econds. The mixture is held an additional 15 minutes at120 F. with stirring and then cooled to 100 F. A mixture of 75 g. of thesodium salt of technical lauryl alcohol sulfate (a white powder) and 7ml. of 3 N sulfuric acid is added in 10 seconds to the silverchlorobromide, stirring continued for one minute and then the mixtureallowed to settle. The supernatant liquid is decanted and replaced by2000 ml. of distilled water containing 4 g. of sodium chloride. Thismixture is stirred for 5 minutes at 100 F., allowed to settle anddecanted again. Two hundred ml. of distilled Water is added to thesilver halide curds and the temperature adjusted to 95 F. This mixtureis vigorously stirred for 10 minutes at 95 F. and then the pH adjustedto 6.li0.1 with aqueous sodium hydroxide solution. The redispersedemulsion is then analyzed for silver halide content calculated as silverbromide and a dispersion made by diluting the appropriate amount withdistilled water such that the dispersion contains 1 mg. calculatedsilver bromide per ml.

Dispersed crystals of silver halide, treated with an appropriate amountof a suitable organic compound are affected by exposure of a portion ofsaid crystals to actinic radiation, e.g., ultraviolet, visible,infrared, x-radiation, etc., to such an extent that at least 20% of theless soluble crystals remain when 90% of the more soluble crystalsdissolve when treated in 10% by weight aqueous sodium thiosulfatesolution. Generally, the presence of solubilizing groups should beavoided in order that reaction products with silver halide will beformed which will significantly reduce the solubility of silver halidegrains in silver halide solvents. There are exceptions to this rule,particularly in the case of suitable compounds containing solubilizinggroups which are compensated for by the presence of insolubilizinggroups, e.g., long chain alkyl groups. The chemical testing forselecting suitable compounds has been found to give absolutecorrelation, i.e., organic compounds which have been subjected to TestsA and B have produced without exception when tested in actualphotographic emulsions. the very effects predicted by said tests.Combinations of the compounds with various basic dyes, including variouscyanine dyes, and Methylene Blue (Colour Index No. 922), Crystal VioletAO (Colour Index No. 681) and Rhodamine 6 GDN Extra (Colour Index No.752) have proven useful additions.

The silver halide need not be combination of silver chloride and silverbromide, but may be silver chloride, silver bromide and other mixedsystems conventional in photographic practice, e.g., silver bromoiodide.While, for rapid processing, a high silver halide to binder ratio ispreferred as described in several of the examples, more conventionalratios can also be used.

In place of part of the gelatin, other natural or syntheticWater-permeable organic colloid binding agents can be used and in somecases such binders can be used alone. Such agents includewater-permeable or watersoluble polyvinyl alcohol and its derivatives,e.g., partially hydrolyzed polyvinyl acetates, polyvinyl ethers andacetals containing a large number of intralinear groups, hydrolyzedinterpolymers of vinyl acetate and unsaturated addition polymerizablecompounds such as maleic anhydride, acrylic and methacrylic acid estersand styrene. Suitable such colloids of the last-mentioned type weredisclosed in US. Patents 2,276,322; 2,276,323 and 2,397,866. The usefulpolyvinyl acetals include polyvinyl acetaldehyde acetal, polyvinylbutyraldehyde acetal and polyvinyl sodium o-sulfobenzaldehyde acetal.Other useful colloid binding agents which can be used include thepoly-N-vinyllactams of Bolton US. Patent 2,495,918, variouspolysaccharides, e.g., dextran, dextrin, etc., the hydrophiliccopolymers in Shacklett US. Pat ent 2,833,650, hydrophilic celluloseethers and esters, and polymers of acrylic and methacrylic esters andamides. Also, it has been found practical to treat silver halide layerson a base material in the essential absence of a binder, e.g., bychemical or vacuum deposition.

The emulsions may optionally contain any of the usual adjuvantscustomarily employed in silver halide system so long as they do notinterfere with the adsorption and complexing action of the essentialingredient of the invention.

The emulsions can be coated on any suitable support, e.g., celluloseesters, cellulose mixed esters; superpolymers, e.g., polyvinyl chloride(co) vinyl acetate, polyvinyl acetals, butyrals; polystyrene;polyamides, e.g., polyhexamethylene adipamide, polyesters, e.g.,polycarbonates, polyethylene terephthalate, polyethyleneterephthalate/isophthalate, esters formed by condensing terephthalicacid and its derivatives, e.g., dimethyl terephthalate with propyleneglycol, diethylene glycol, tetramethylene glycol,cyclohexane-1,4-dimethanol (hexahydro-p-xylene dialcohol); paper, metal,glass, etc.

As disclosed earlier, the desirable concentration of the selectedorganic compound depends on many factors such as the size and solubilityof the organic compound, the nature of its reaction with silver halide,the size and nature of the silver halide crystals, the presence of othermaterials which may react with or be adsorbed to the surface of thesilver halide, etc. In Example I below, a large number of organiccompounds are disclosed which were tested in a dispersion of silverhalide crystals wherein the average grain size was 035a (micron) indiameter therefore about 0.43n in volume, assuming cubic grains. Thesilver halide comprised 70 mole percent silver chloride and 30 molepercent silver bromide, with a specific density of about 5.7 g./cc. or5.7 l0- g./u The weight per individual crystal or grain is O.043,u. 5.7g./,u =O.25 lO- Assuming a molecular weight of 157 for the mixedAgCl-AgBr crystals, and dividing this number by the weight per grain,gives 157 g./mole+().25 l0- g.=6.3 l0 grains/mole. The area of a cubicgrain of 0.35 diameter=6 .35 =.74;t which multiplied by the 6.3)(10grains per mole, gives a molar surface area of 4.6x 10 or 45x10 squareangstroms.

A particularly preferred organic compound is 2-mercapto-4-phenylthiazole (hereinafter to be referred to as MPT). Thiscompound is disclosed and claimed in my copending application Ser. No.317,824, filed October 21, 1963, entitled Photographic Compositions,Layers and Elements which is a continuation-in-part of this application.Assuming that a single molecule of MPT could occupy an area of 28 squareangstroms, it would require 1.5 10 molecules to occupy a molar surfacearea of silver halide. With a molecular weight of 193, this wouldrequire of MPT to insolubilize one mole of the silver halide.

More significantly, as disclosed in Procedure A in a photographicemulsion coated on a film base support, it was found that 0.4 g. of MPTper mole of silver halide gave optimum results. This compares moreclosely with the theoretically determined amount of MPT required tocover the silver halide surface.

As shown in various examples below, e.g. Example I, elements suitablefor this novel process can be prepared by treatment with an appropriatesubstituted thiourea compound. In this embodiment, the silver halidecrystals near the surface of the coated emulsion stratum are in contactwith a higher concentration of the organic compound. Crystals fartherfrom the surface, are treated with less of the organic compound and, ifthe rate of diffusion is sufiiciently slow, there may be considerablyless of the organic compound (even approaching zero) reacting With thelower than with the surface silver halide crystals. In such elements,satisfactory results might be obtained with only a fraction, e.g.,one-half, of the amount of the organic compound theoretically calculatedas required to just cover the surface of a mole of the silver halidecrystals.

The invention will be further illustrated by but is not intended to belimited to the following examples.

EXAMPLE I Tests A and B have been described earlier as procedureswhereby one can determine whether or not a given organic compound issuitable for use according to the process of this invention. Many of thecompounds which were indicated as suitable according to the screeningprocedures of both tests, have been incorporated into that there iscomplete testing correlation in that any compound which was foundsuitable according to the photographic test to be described in the nextparagraph was also found suitable according to Tests A and B.

TEST C A 0.5 ml. portion of the insolubilized dispersion prepared inTest A under safelight conditions is placed in a 12 x 75 mm. Pyrex testtube three inches from a No. 2 reflectoflood lamp. This insolubilizeddispersion is exposed to the lamp for up to 10 minutes. A controlconsisting of another 0.5 ml. portion of the insolubilized silver halidedispersion from Test A is taken under safelight conditions. Two-tenthsof a milliliter of 10% aqueous sodium thiosulfate is added to each ofthe dispersion samples taken and compared under safelight conditions.Any reduction in turbidity of the dispersion exposed to thereflectofiood lamp compared to the unexposed control after treatmentwith aqueous sodium thiosulfate solution shows that photosolubilizationoccurs.

Tests A, B and C were all conducted using Silver Halide Dispersion I,the preparation of which was given immediately following the descriptionof the procedure for Test B. To determine and approximate minimumconcentration of the organic compound required to effectinsolubilization of silver halide in the presence of an aqueous solutionof sodium thiosulfate, the qualitative procedure of Test A was repeatedin a more quantitative manner, using a ripened, washed and redispersed(but not chemically sensitized) gelatino-silver chlorobromide emulsionas described in Example I of assignees copending application, Nottorf,U.S. Serial No. 94,989, filed March 13,

1961. This emulsion is designated in the table below as Dispersion II,and was made as follows:

A lithographic emulsion having a sliver halide composition of 30 molepercent AgBr and 70 mole percent AgCl and having grams of gelatinpresent per mole of silver halide for the steps of precipitation andripening was freed of unwanted, soluble, by-product salts by acoagulation and wash procedure as taught in Waller et al., US. Patent2,489,341, wherein the silver halide and most of the gelatin werecoagulated by an anionic wetting agent, sodium lauryl sulfate, using anacid coagulation environment. Following the washing step, the emulsioncoagulate was redispersed in water together with 47 grams of additionalbulking gelatin.

Tlu'oui'ca Derivatives The silver halide pliotosoluble elements of thisinvention differ from conventional silver halide emulsions containingantifogging agents in that the insolubilizing compounds used in thephotosoluble elements are present in substantially greater thanfog-inhibiting amounts, the latter amounts being the maximum quantitywhich provides low fog without serious loss in speed and photographicquality. For this reason it is not practical to use photosolubleelements in place of ordinary silver halide photographic materials. Whenphotosoluble elements are exposed and processed normally, developmentproceeds slowly and incompletely to give a negative silver image havingmuch less speed and lower density. In addition, fixing is slower and maybe incomplete for practical fixing Test Results with Dispersion I Gms.Compound to Insolubilizo Dispersion 11 Compound Insolubili- ChemicalPhotosolu- Containing zation Solubilizabilization 111g. Silver Test Ation Test B Test 0 Halide 1. 'lhioacetanilide Insoluble.-- 0,0025 2.Thiobenzanilide. .d0 0,002 3. 'lhiourea 4. N-phenylthiourea 0,002 5.1-(l-naphthyl)-2thiourea 0,0005 0. 1,1-diphenyl-2-thiourea 0,001 7.l-ethyl-l-(l-naphthyl)2-thiourea. 0, 0006 8. 1,3-di-n-buty1 2-thiourea0, 0002 9. l-etl1yl-3-phenyl-2thiourea. 0,0035 10. Thiocarbanilide 0,0000 11. 2,2'-diethylthiocarbanilide. 0, 0006 12. 1,3-dibcuzyl-2thiourea0,0002 13. 1,3 di-(1-naphthyl)-2thiourea 0,0035

14. '1hiosemiearbuzide 0,01 15. 4-pheuyHi-thioseiuicarbazide 0,0002 10.1,5-diphenyl-H-thiocarbohydrazide 0, 0009 17. Diphenylthioearbazone-0,003 18. a-Mereapto aeetanllide 0,001 10. 1,3-diallyl-2-tliiourea.0,005 20. l-ullyl -phonyl-2-tl1iou 0,005 21.1.3-(1i-n-oetyl-fl-thiourean 0,004 22. Thioaeetamide 23.l-acetyl-Z-thiourea l Procedure A.An emulsion, as described inDispersion II, Example I was brought to 2320 g, by addition of water andthe temperature adjusted to 120 F. Fourtenths of a gram of MPT(2-mercapto-4-phenylthiazole) was added per mole of silver halide from a1% by weight ethanol solution. Chrome alum hardener was added and theemulsion was diluted with water to a total weight of 2334 g. per mole ofsilver halide. This emulsion was applied at a coating weight of 46 mg.of silver per square decimeter on 0.004 inch thick polyesterphotographic film base as described in Example I. The coating, afterimagewise exposure, showed a greater rate of fixing in a 1.0 N (0.5molar) aqueous sodium thiosulfate in exposed areas than in the unexposedareas so as to form a positive silver halide image. Subsequent flashingto white light, followed by treatment with a reducing agent (aconventional photographic developing solution containing 1-phenyl-4-methyl-3-pyrazolidone and hydroquinone), resulted in theformation of a positive image of metallic silver.

EXAMPLE II Example I was repeated except that other thiourea compounds,in the same amounts by weight, were examined in place of MPT. Afterphotographic processing as described in Procedure A, the opticaldensities of completely unexposed areas and heavily exposed areas of thefilm were determined using a Western Electric RA-lOO-C Densitometer.

times. Thus, photosoluble elements require longer conventionalprocessing times and give slower speed, inferior quality images whencompared to ordinary silver halide photographic elements.

The novel photographic compositions of this invention have numerousadvantages. A primary advantage is the simplicity of their preparation.They can be exposed and processed to images under ordinary room lightconditions.

The photographic processes applicable to the compositions of theinvention likewise have advantages over previously known systems basedon selective reduction of exposed silver halide for forming eitherdirect positive or negative images without resorting to the specialeffects and sensitizing procedures previously used for preparing suchimages. In addition, since image formation does not require selectivereduction, this present process is not limited to the use of certainphotographic developing agents but may be accomplished by using a widerange of reducing agents. Many such compounds are of very low cost andcan be used to form images of much higher covering power than customary,thus effecting important economies in processing, as well as greatlyincreasing the ethciency of the silver image with a resultant increasein sensitivity.

Another advantage of this invention is that it provides new elements forforming silver images that do not require special equipment but insteadcan be used with conventional equipment and apparatus. A furtheradvantage is that the elements can be used successfully by photographictechnicians and photographers of ordinary skill. A still furtheradvantage is that the elements can be processed with conventionalreducing agents, e.g., developers and fixing agents. A still furtheradvantage is that the new elements can be used to produce images withoutselective reduction. Still additional advantages will be apparent fromthe above description of the invention.

This application is a continuation-in-part of my application Ser. No.158,132, now abandoned, filed December 8, 1961 for PhotographicCompositions and Processes.

I claim:

1. A photographic silver halide emulsion layer comprising, beforeexposure to actinic light, silver halide crystals and associatedtherewith in substantially greater than fog-inhibiting amounts a silvermercaptide of a substituted thiourea compound containing at least one %Hgroup, the silver mercaptide being of lower solubility in water thansilver chloride, the silver halide crystals so associated with thesilver mercaptide being less soluble in 10% aqueous sodium thiosulfatethan untreated silver halide crystals at a predetermined pH and thesubstituted thiourea compound being present in such an amount, in termsof the ratio of its weight to the surface area of said silver halidecrystals, that when admixed in such ratio with an aqueous silverchlorobromide (70/30 mole percent) gelatin emulsion containing 57 g. ofgelatin per mole Ag and .57 mg. of Ag per ml., and said silverchlorooromide dispersion is treated with 10%, by weight, aqueous sodiumthiosulfate (so that the resulting mixture contains 0.29 mg. of silverand 1 mg. of sodium thiosulfate), at least three times the amount ofsilver chlorobromide remains undissolved as compared with similardispersion successively treated with a 5%, by weight, aqueous sodiumhypochlorite and by weight, aqueous sodium thiosulfate (so that theresulting mixture contains 0.29 mg. of silver, 25 mg. of sodiumhypochlorite and 100 mg. of sodium thiosulfate), after vigorousagitation of the dispersions for 30 seconds at 25 C.

2. A photographic layer according to claim 1 wherein said emulsion layeris a gelatino-silver chlorobromide emulsion.

3. A photographic layer according to claim 1 wherein said emulsion layeris a gelatino-silver chlorobromide emulsion and the gelatin/silverhalide ratio is from 3:1 to 1:30 by weight.

4. A photographic silver halide emulsion layer according to claim 1,wherein said compound is N-phenylthiourea.

5 A photographic silver halide emulsion layer according to claim 1,wherein said compound is 1,3-dibenzylthiourea.

6. A photographic element comprising a support bearing a photographicsilver halide emulsion layer as defined in claim 1.

References Cited in the file of this patent UNITED STATES PATENTS2,214,446 Albers et a1. Sept. 10, 1940 3,008,829 Clementi et al. Nov.14, 1961 3,046,130 Dersch et al. July 24, 1962 3,063,837 Lassig et al.Nov. 13, 1962 3,080,230 Haydn et al. Mar. 5, 1963 OTHER REFERENCES Land:A New One-Step Photographic Process, Journal of the Optical Society ofAmerica, Vol. 37, No. 2, pages 61 to 77, February 1947.

Keelan: The Journal of Photographic Science, vol. 5, November/December1957, pages l44 and 145.

Faerman et al.: Chem. Abst., vol. 52, No. 2, Col. 937d; January 25, 1958(abstract of Otdel Chim. Nauk 5, 107- 13 (1957)).

1. A PHOTOGRAPHIC SILVER HALIDE EMULSION LAYER COMPRISING, BEFORE EXPOSURE TO ACTINIC LIGHT, SILVER HALIDE CRYSTALS AND ASSOCIATED THEREWITH IN SUBSTANTIALLY GREATER THAN FOG-INHIBITING AMOUNTS A SILVER MERCAPTIDE OF A SUBSTITUTED THIOUREA COMPOUND CONTAINING AT LEAST ONE -SH GROUP, THE SILVER MERCAPTIDE BEING OF LOWER SOLUBILITY IN WATER THAN SILVER CHLORIDE, THE SILVER HALIDE CRYSTALS SO ASSOCIATED WITH THE SILVER MERCAPTIDE BEING LESS SOLUBLE IN 10% AQUEOUS SODIUM THIOSULFATE THAN UNTREATED SILVER HALIDE CRYSTALS AT A PREDETERMINED PH AND THE SUBSTITUTED THIOUREA COMPOUND BEING PRESENT IN SUCH AN AMOUNT, IN TERMS OF THE RATIO OF ITS EIGHT TO THE SURFACE AREA OF SAID SILVER HALIDE CRYSTALS, THAT WHEN ADMIXED IN SUCH RATIO WITH AN AQUEOUS SILVER CHLOROBROMIDE (70/30 MOLE PER CENT) GELATIN EMULSION CONTAINING 57G. OF GELATIN PER MOLE AG AND .57 MG. OF AR PER ML., AND SAID SILVER CHLOROBROMIDE DISPERSION IS TREATED WITH 10%, BY WEIGHT, AQUEOUS SODIUM THIOSULFATE (SO THAT THE RESULTING MIXTURE CONTAINS 0.29 MG. OF SILVER AND 100 MG. OF SODIUM THIOSULFATE), AT LEAST THREE TIMES THE AMOUNT OF SILVER CHLOROBROMIDE REMAINS UNDISSOLVED AS COMPARED WITH SIMILAR DISPERSION SUCCESSIVELY TREATED WITH A 5%, BY WEIGHT, AQUEOUS SODIUM HYPOCHLORITE AND 10%, BY WEIGHT, AQUEOUS SODIUM THIOSULFATE (SO THAT THE RESULTING MIXTURE CONTAINS 0.29 MG. OF SILVER, 25 MG. OF SODIUM HYPOCHLORITE AND 100 MG. OF SODIUM THIOSULFATE), AFTER VIGOROUS AGITATION OF THE DISPERSIONS FOR 30 SECONDS AT 25*C. 